This invention relates to an auxiliary hydrostatic front wheel drive system for a vehicle and more particularly to a system which utilizes variable displacement hydrostatic drive motors.
Drive systems of the general type described herein are disclosed in U.S. Pat. No. 3,458,005 to Malm et al, U.S. Pat. No. 3,480,099 to Nighswonger et al, and U.S. Pat. No. 3,481,419 to Kress et al. While these systems would appear to be effective in the use for which they are designed, it is to be noted that the systems thereof utilize fixed displacement hydraulic motors for driving the front wheels of the vehicle in the auxiliary drive system thereof. Thus, there is no teaching of a system which is specifically to be used in combination with fluid drive motors of the variable displacement type. Of particular interest herein is an association of the displacements of the axuiliary drive motors in response to a primary drive condition of the vehicle. It will be understood, of course, that none of the three references cited above are concerned with such a system.
Also of interest are U.S. Pat. No. 3,354,978 to Budzich, disclosing a hydraulic motor responsive to a pump, U.S. Pat. No. 3,680,652 to Greene, wherein the displacement of a hydraulic motor is varied by a hand lever, and U.S. Pat. No. 3,827,528 to Shaffer, wherein the displacement of a motor is responsive to motor line pressure.
Of more general interest in this area are U.S. Pat. No. 2,818,699 to Clemson, U.S. Pat. No. 3,153,908 to Lawrence, U.S. Pat. No. 3,184,994 to Stahl, U.S. Pat. No. 3,186,506 to Leach, U.S. Pat. No. 3,255,840 to Tangen, U.S. Pat. No. 3,272,576 to Budzich, U.S. Pat. No. 3,272,279 to Budzich, U.S. Pat. No. 3,302,741 to Brazuk, U.S. Pat. No. 3,354,977 to Swift, U.S. Pat. No. 3,391,753 to Anderson, U.S. Pat. No. 3,361,223 to Baver, U.S. Pat. No. 3,415,334 to Vriend, U.S. Pat. No. 3,469,648 to Cannon, U.S. Pat. No. 3,477,547 to Kress et al, U.S. Pat. No. 3,493,067 to Rumsey, U.S. Pat. No. 3,522,861 to Middlesworth et al, and U.S. Pat. No. 3,579,988 to Firth et al.